1. Field of the Invention
The present invention relates to the use of a dewaxing aid in a process for the solvent dewaxing of a wax-containing hydrocarbon oil. More particularly, this invention relates to a solvent dewaxing process containing a dewaxing aid comprising a mixture of (A) an .alpha.-olefin copolymer and (B) an olefin-vinyl acetate copolymer. Still more particularly, this invention relates to an improvement in a solvent dewaxing process using a dewaxing aid wherein the improvement comprises a dewaxing aid comprising a mixture of (A) an .alpha.-olefin polymer having a molecular weight ranging from about 10,000 to 1,000,000 and a wide molecular weight distribution exceeding the range of from about 10,000 to 1,000,000 but falling within the range of from about 2,000 to 3,000,000 and (B) an olefin-vinyl acetate copolymer having a vinyl acetate content ranging from about 15-40 wt. % and having an average molecular weight of from about 50,000 to 1,000,000.
2. Description of the Prior Art
Waxes in wax-containing hydrocarbon oils are removed therefrom by chilling the oil to precipitate out the wax and then separating the solid wax particles from the dewaxed oil by filtration or centrifugation. Industrial dewaxing processes include press dewaxing processes wherein the wax-containing oil, in the absence of solvent, is chilled to crystallize out the wax particles which are then pressed out by a filter. In general, only light hydrocarbon oil fractions (paraffinic fractions) obtained by vacuum distillation are treated by the press dewaxing processes due to viscosity limitations. More widely used are solvent dewaxing processes wherein a waxy oil is mixed with a solvent and then chilled to precipitate the wax as tiny particles or crystals thereby forming a slurry of solid wax particles and a dewaxed oil containing dewaxing solvent. The slurry is then fed to a wax filter wherein the wax is removed from the dewaxed oil and dewaxing solvent. Solvent dewaxing processes are used for heavier oil fractions such as lubricating oil fractions and bright stocks. Typical dewaxing solvents include low boiling point or autorefrigerative hydrocarbons such as propane, propylene, butane, pentane, etc., ketones such as mixtures of acetone and MEK or MEK and MIBK as well as mixtures of ketones and aromatic hydrocarbons such as MEK/toluene and acetone/benzene.
One of the factors tending to limit the capacity of a solvent dewaxing plant is the rate of wax filtration from the dewaxed oil, which in turn is strongly influenced by the crystal structure of the precipitated wax. Although the crystal structure of the precipitated wax is influenced by various operation conditions in the dewaxing process, for any given feed it is most strongly influenced by the chilling conditions. The size and crystal structure of the precipitated wax, occlusion of oil in the wax crystal and the condition of the oil left in the crystal are extremely varied and depend on the wax composition and precipitation conditions. These conditions also affect the filtration rate of the dewaxed oil from the wax and the yield of dewaxed oil. In some cases, most notably when the waxy oil is a bright stock, the wax crystals are of an extremely fine size and not all are separated by filtration, but some leave the filter with the dewaxed oil component which creates an objectionable haze in the oil. Also, in some dewaxing processes too-rapid chilling of the waxy oil results in a so-called shock chilling effect yielding extremely fine sizes of wax crystals having poor filtration characteristics exhibited as a reduction of the filtration rate, decreased yield and increased pour point of the dewaxed oil. This phenomena often happens in autorefrigerant dewaxing processes wherein the waxy oil is chilled by the latent heat of vaporization of an autorefrigerant such as liquid propane.
One way of increasing the wax filtration rate is to add a dewaxing aid to the wax-containing oil. Well known in the art are dewaxing aids such as .alpha.-olefin copolymers and mixtures of materials such as a mixture of an ethylene-vinyl acetate copolymer and an unsaturated ester of an aliphatic alcohol having from 2 to 20 carbon atoms with acrylic or methacrylic acid. However, these dewaxing aids are not terribly efficient, necessitating therefore relative high concentrations of same in the oil. This is especially true when a residual oil raffinate such as a bright stock is solvent dewaxed wherein a portion of the wax is precipitated as crystals so fine that they pass through filter cloths thereby creating a haze in the dewaxed oil which greatly reduces the commercial value of same. Haze may also appear when the dewaxed oil is allowed to stand at room temperature for a long time. In order to prevent the occurrence of haze, it is sometimes necessary to filter the dewaxed oil through a sintered metal filter or a polyvinyl alcohol filter after the dewaxing step, thereby adding to the complexity and cost of the dewaxing process. It has now been found that in order for a dewaxing aid in a liquid mixture of dewaxing solvent and waxcontaining oil to avoid platelike, needle or amorphous crystals having relatively poor filtration characteristics and instead to form eutectic crystals with the wax that is precipitated via gradual cooling, which crystals are relatively uniform, spherical wax crystals having a size of from about 20 to about 100 microns which possess superior filtration characteristics resulting in faster wax filtration rates, it is necessary for the dewaxing aid to have a high average molecular weight and a wide molecular weight distribution.